1. Field of the Invention
Embodiments relate to the field of photographic flashes and in particular to efficient flash termination and charging.
2. Related Art
Photographic flashes use a high percentage of the battery power available to modem cameras. Despite the level of commercial interest in photography, electronic flashes remain highly inefficient. In a typical flash, only 30 percent of the energy drained from the battery reaches the flash capacitor.
FIG. 1 is a schematic diagram of a typical flash system. Capacitor 114 is charged from battery 108 by charge circuit 116. To make a flash, controller 121 closes switch 120 and sends trigger signal 119 to cause trigger circuit 118 to send a pulse through electrode 112 of flash tube 110. Trigger signal 119 partially ionizes the gas in flash tube 110; capacitor 114 then discharges through the gas, causing a flash of light energy to be radiated. The flash stops when the voltage on capacitor 114 falls below a threshold or switch 120 is opened.
Prior-art photo flashes use minority-carrier semiconductor switching devices, also known as conductivity-modulated devices or bipolar devices, as switch 120. Use of such devices incurs problems with timing uncertainty and parasitic power losses, due to a turn-off delay, of typically many microseconds, that depends on minority carrier storage and recombination times. Some of these flash systems emit multiple flashes of light for one picture; however, timing uncertainty lowers performance or renders the circuits complex.
FIG. 2 is a schematic diagram of flyback converter charge circuit 200, typical in photographic flashes. FIG. 3 is a timing diagram. Current flows through primary winding 242 of coupled inductor 241 when drive circuit 244 turns on transistor 246, completing a circuit through primary 242 from battery 108. Transistor gate voltage and primary voltage are shown by traces 301 and 302, respectively, in FIG. 3; trace 303 shows the drain voltage of transistor 246. When drive circuit 244 turns off transistor 246, mutual inductance generates current in secondary winding 243. Voltage across secondary 243 is shown by trace 304 in FIG. 3. Diode 248 allows current to flow from secondary 243 into capacitor 114, and not back out. Thus, the circuit charges capacitor 114 over many cycles.
Typical flyback converters have inefficient coupled inductors that waste power, and that can create overshoot voltages at transistor 246, potentially damaging it. Also, the current drained from battery 108 may have steep spikes and dips, lowering battery life.
FIGS. 4A and 4B are cross-section illustrations of the winding of a typical coupled inductor. Primary winding 242 is wound around plastic bobbin 460; then, insulation 468 is placed over winding 242; finally, layers of secondary winding 243 are wound over insulation 468. Ferrite core 250 with axis 464 is made in two halves, 455 and 456. Plastic bobbin 460 supports windings 242 and 243, shown with an xe2x80x9cX.xe2x80x9d
Typical coupled inductors suffer from primary-winding leakage inductance and skin effect. Leakage inductance is caused by poor magnetic-field coupling between primary winding 242 and secondary winding 243. Primary leakage inductance causes overshoot voltages that can damage switching transistor 246. Skin effect causes energy losses by increasing the impedance of the windings at high frequencies. Skin effect dominates the resistive losses in primary windings that are made from thick wire.
Many coupled inductors are wound on iron cores, rather than on core materials that do not easily saturate. Such an inductor reaches saturation while the current in the primary winding is still increasing, and wastes energy that cannot be stored in the core""s magnetic field.
FIG. 5 is taken from FIG. 5 of U.S. Pat. No. 5,430,405, a schematic diagram of a coupled-inductor charging circuit and driver. A typical problem with such circuits is that, as capacitor 114 approaches higher charge voltages, the cyclical action of circuit 500 speeds up to drive higher voltage into capacitor 114, causing the current drained from battery 108 to increase beyond a limit where the battery may be damaged, and thus shortening battery life.
Thus, it would be desirable to have a flash system that saves battery energy, extends battery life, and enhances flash performance by controlling flash timing accurately, with little energy loss, and by including a charge circuit with an efficient coupled inductor that also limits overshoot voltages and battery-current spikes, and that has a switching rate controlled by a drive circuit that limits the amount of current drained from the battery and uses energy-efficient components.
A more detailed background of related flash and charge circuits is included in Appendix A.
In accordance with the present invention, energy efficiency of a photographic flash is improved by provision of several unique circuits that significantly increase the efficiency of the flash. Efficiency, measured by energy stored on the flash capacitor divided by energy drained from the battery, is conserved by precisely timed flash termination, a low-loss flyback converter, a high-efficiency coupled inductor, and a battery-saving charge circuit, including a new drive. When the several improvements are combined, total energy efficiency is improved from a nominal 30-percent efficiency to close to 90-percent efficiency.
In some embodiments, a majority-carrier switching-device circuit controls flash termination, starting and stopping the flow of current from the flash capacitor through the flash tube. This circuit eliminates the problems of timing uncertainty and transient energy dissipation, which are associated with previous designs, thereby making possible more precisely timed flashes, including multiple flashes. Thus, energy is not wasted by being dumped from the flash capacitor or in transient energy dissipation. The disclosed flash-control method may also be used in conjunction with a through-the-lens (TTL) exposure control that determines how much flash energy is needed for capture of a given image, and that commands the flash control to deliver only that much flash energy, thereby further saving energy.
Some embodiments use a high-efficiency coupled inductor to save energy during charging of the flash. This coupled inductor makes use of both an overlapping winding configuration and multiple primary winding strands. Multiple primary strands lower energy losses caused by skin effects. The winding configuration enables the primary and secondary windings to share the magnetic field of the core more efficiently, thus lowering primary leakage inductance, which is another source of energy loss. Lower primary leakage inductance also results in smaller voltage spikes during turn-off of the primary winding.
A charge circuit that uses the high-efficiency inductor does not require an active snubber to damp voltage spikes. Omitting the snubber circuitry saves energy. Several embodiments of such an energy-saving charge circuit are disclosed; each has simple and efficient damping circuits that control effectively the reduced overshoot voltages and that smooth battery current drain. Because overshoot voltages are controlled, the field-effect transistor (FET), which is used to drive the charge circuit, can also be small and energy efficient. The circuit extends battery life by smoothing out peaks in the battery-current drain.
Some embodiments of the present invention include a new drive circuit that keeps battery-current drain below a threshold value, thus further extending battery life. Some embodiments of the drive circuit save additional energy by using discrete transistor circuits rather than operational amplifiers.
By combining several novel circuits and devices, the various embodiments of the resent invention improve overall energy efficiency.